The present invention relates to turbine systems. More particularly, the invention relates to components of such turbine systems. Still more particularly, the invention relates to turbine components formed from a niobium-based refractory metal intermetallic composite. Finally, the invention relates to environmentally resistant coatings for such turbine components.
Turbine systems, such as, but not limited to, aeronautical turbines, land-based, turbines, marine-based turbines, and the like, and their components (hereinafter referred to as xe2x80x9cturbine componentsxe2x80x9d) have typically been formed from nickel (Ni) based materials, which are often referred to as Ni-based superalloys. Turbine components formed from these Ni-based superalloys exhibit desirable chemical and physical properties under the high temperature, high stress, and high-pressure conditions generally encountered during turbine operation. For example, the highest surface temperatures of state-of-the-art jet engine turbine airfoils reach as high as about 2100xc2x0 F. (about 1150xc2x0 C.), or about 85% of the melting temperature (Tm) of most of the Ni-based superalloys.
To date, the Ni-based superalloys have provided the desired level of performance for turbine system applications, causing the development of such Ni-based superalloys to be widely explored. As a result of such extensive study, the field has matured and few significant improvements have been realized in this area in recent years. In the meantime, efforts have been made to develop alternative turbine component materials.
These alternative materials include niobium (Nb) based refractory metal intermetallic composites (hereinafter referred to as xe2x80x9cNb-based RMICsxe2x80x9d). Most Nb-based RMICs have melting temperatures of greater than about 3100xc2x0 F. (about 1700xc2x0 C.). If Nb-based RMICs can be used at about 80% of their melting temperatures, they will have potential use in applications in which the temperature exceeds the current service limit of Ni-based superalloys.
Nb-based RMICs comprising niobium (Nb), silicon (Si), titanium (Ti), hafnium (Hf), chromium (Cr), and aluminum (Al) are among the materials that have been proposed for turbine component applications in which Ni-based superalloys are presently used. These Nb-based RMICs exhibit a high temperature capability which exceeds that of the Ni-based superalloys that are currently used in such applications. Exemplary Nb-based RMICs are described by Jackson and Bewlay (U.S. Pat. Nos. 5,932,033 and 5,942,055), and more recently by Jackson, Bewlay, and Zhao in U.S. Patent applications titled xe2x80x9cNiobium-Silicide Based Composites Resistant to High Temperature Oxidationxe2x80x9d (Ser. No. 09/735,767; filed Dec. 13, 2000) and xe2x80x9cNiobium-Silicide Based Composites Resistant to Low Temperature Pestingxe2x80x9d (Ser. No. 09/735,769; filed Dec. 13, 2000).
Although the Nb-based RMICs show potential for use as next-generation turbine components having service temperatures that are significantly greater than those of current Ni-based superalloy components, oxidation of such turbine components remains a concern. At temperatures in the range between about 2000xc2x0 F. and about 2500xc2x0 F. (between about 1090xc2x0 C. and about 1370xc2x0 C.), refractory materials can undergo rapid oxidation. While a slow-growing oxide scale can form on Nb-based RMIC""s at this temperature, it is typically not a protective oxide scale. Another type of oxidation known as xe2x80x98pestingxe2x80x99 occurs at intermediate temperatures (e.g., between about 1400xc2x0 F. and about 1800xc2x0 F.). Pesting is a phenomenon that is characterized by the disintegration of a material into pieces or powders after exposure to air at intermediate temperatures. Refractory metals, particularly molybdenum, exhibit poor resistance to pesting oxidation.
While significant progress has been made in improving the oxidation performance of Nb-based RMICs, it is desirable to provide coatings for turbine components fabricated from these materials in order to ensure long lifetimes at service temperatures of 2000xc2x0 F. to 2500xc2x0 F. Therefore, what is needed is a turbine system having Nb-based RMIC components having coatings that will provide increased resistance of the components to oxidation at temperatures in the range between about 2000xc2x0 F. and about 2500xc2x0 F. and increased resistance to pesting at temperatures between about 1400xc2x0 F. an about 1800xc2x0 F. What is also needed is an environmentally resistant coating for Nb-based RMICs, which will enhance oxidation resistance at high temperatures and pesting resistance at intermediate temperatures.
The present invention meets these needs and others by providing a turbine system that includes Nb-based RMIC components having coatings that increase oxidation resistance at high temperatures and resistance to pesting at intermediate temperatures. The present invention also provides an environmentally resistant coating for Nb-based RMICs that exhibit improved oxidation resistance at high temperatures and resistance to pesting at intermediate temperatures. In addition, methods for making a coated Nb-based RMIC turbine component and coating a Nb-based RMIC are also disclosed.
Accordingly, one aspect of the present invention is to provide a turbine system having at least one turbine component. The turbine component comprises: a niobium-based refractory metal intermetallic composite (Nb-based RMIC) comprising titanium, hafnium, silicon, chromium, and niobium; and an environmentally resistant coating disposed on a surface of the niobium-based refractory metal intermetallic composite, the environmentally resistant coating comprising silicon, titanium, chromium, and niobium.
A second aspect of the invention is to provide an environmentally resistant coating for a niobium-based refractory metal intermetallic composite (Nb-based RMIC) substrate. The environmentally resistant coating comprises between about 43 and 67 atomic percent silicon; between about 2 and about 25 atomic percent titanium; between about 1 and about 25 atomic percent chromium; and a balance of niobium.
A third aspect of the invention is to provide a turbine system having at least one turbine component. The turbine component comprises: a niobium-based refractory metal intermetallic composite (Nb-based RMIC), said niobium-based refractory metal intermetallic composite comprising titanium, hafnium, silicon, chromium, and a balance of niobium; an environmentally resistant coating disposed on a surface of the niobium-based refractory metal intermetallic composite substrate; and a thermal barrier coating disposed on an outer surface of the environmentally resistant coating. The environmentally resistant coating comprises between about 43 and 67 atomic percent silicon; between about 2 and about 25 atomic percent titanium; between about 1 and about 25 atomic percent chromium; and a balance of niobium.
A fourth aspect of the invention is to provide a method of making a turbine component comprising a niobium-based refractory metal intermetallic composite (Nb-based RMIC) and having an environmentally resistant coating disposed on the surface of the component, The environmentally resistant coating comprises silicon, titanium, chromium, and niobium. The method comprises the steps of providing a niobium-based refractory metal intermetallic composite substrate formed into the turbine component and depositing the environmentally resistant coating onto the surface of the component.
Finally, a fifth aspect of the present invention is to provide a method of coating a niobium-based refractory metal intermetallic composite (Nb-based RMIC) substrate with an environmentally resistant coating. The environmentally resistant coating comprises silicon, titanium, chromium, and a balance of niobium. The method comprises the steps of providing a niobium-based refractory metal intermetallic composite substrate and depositing the environmentally resistant coating onto the surface of the niobium-based refractory metal intermetallic composite substrate.
These and other aspects, advantages, and salient features of the invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.